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Chiaia Development of the muscular system


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What does the dorsal somatic layer give rise to?
It gives rise to CT and some vascular smooth muscle.
What does the ventral splanchnic layer give rise to?
It gives rise to smooth muscle of gut and cardiac muscle.
Describe somitogenesis. What forms? Where do they form?
The paraxial mesoderm condenses in response to molecular signals.
These condensations form indestinct spherical aggregates called somitomeres along the rostro/caudal axis of the embryo.
Somitomeres furhter differentiate into distinctly segmented aggregates called somites.
After which somitomere are all the rest somites?
The 7th somitomere.
Describe somitomeres. How many form? In which direction do they form? At what point does somite differentiation begin?
Somitomeres are loose masses of paraxial mesoderm derived cells that form circular aggregates along each side of the neural tube towards the end of the third gestational week.
Approximitely 50 pairs of somitomeres begin developing in the cranial region, continuing in a caudal direction untill week 4.
When approximately 20 pairs of somitomeres have formed the caudal somitomere behind somitomere 7 begin to furhter develop into well segmented somites.
What causes somitomeres to compact and segment?
Expression of specific genes and adhesion proteins results in their compaction.
From what end is each new somite formed from the somitomere?
From the anterior end of the somitomere.
What do the first 7 pairs of somitomeres give rise to?
The first seven somitomeres give rise to the striated muscles of the face, jaws, and throat.
What do the caudal somites give rise to (3 regions)? What do each of these give rise to?
Scleretome - vertebral column
Myotome - axial muscles
Dermatome - overlying dermis
What initiates somite segmentation?
Tyrosine kinase receptor (Ephrin/Eph) innitiates somite segmentation.
For what is the cranial caudal specificaiton important? For development of what is it unimportant? What does this mean?
Cranio-caudal specification is important for the final determination of skeletal components, but is less important for muscle development. This means that final specification of muscles is provided by local cues from connective tissues.
What determines the cranial caudal commitment of somites?
The expression of Hox genes.
When does commitment of cells within a given somite occur?
It occurs only after the somite is formed.
What is a dermomyotome?
A transient epithelial structure that formes the dorsal compartment of each somite.
Describe the formation of the somitocoel.
As somites form and segregate the paraxial mesodermal mesenchymal cells undergo epitheliaization and transform into a shell of epithelial cells surrounding a central lumen (somitocoel).
What region of the somite wall forms the sclerotome? What does the sclereotome eventually form?
The ventral region of the somite wall forms the sclereotome which forms the cartilage ribs and vertebrae of the axial skeleton.
What does the dermamyotome differentiate into and in what region are they found?
A dorsal region called the dermatome which fomres the dermis of the back and a ventral region which forms the muscle tissue.
What causes cells within the dermomyotome to become committed to a myogenic lineage? What happens next?
Two myogenetic regulatory factors MyoD and Myf5, induce muscle cell formation. Once committed to the myogenic lineage, myogenic precursors delaminate from the medial and lateral edge of the dermamyotome and migrate underneath the dermamyotome layer where they aggregate to form myotomes.
What do myotmes parcellate into (two regions), and where are they located?
The epaxial region (dorsomedial), and the hypaxial (ventrolateral).
What do myoblasts derived from the epaxial myotome form?
The extensor muscles of the neck and vertebral column. (innervated by the dorsal primary ramus of the spinal nerves.
What do myoblasts derived from the hypaxial region form?
The muscles of the body wall, limbs, and tongue. These are innervated by the vental primary ramus.
What do myoblasts derived from the pharynx form?
The muscles of mastication, faxial expression, pharynx and larynx.
What do axial trunk muscles develop from?
It develops from myoblast that remains in the myotomes.
How do muscle cell precursors become commited to their myogenic fate in the dermamyotome? How do they work?
Through the action of MyoD and Myf5. These factors act on muscle precursors by activating muscle-specific genes which
1) induce specification to the myogenic fate.
2) promote the fusion of myogenic cells to form muscle fibers
3) direct the organization of contractile proteins within the maturing muscle fibers.
What are trunk muscles formed from? What are they innervated by?
Trunk muscles are formed from myoblasts that remain in the epaxial and hypaxial regions of the the somites that stregch around the body wall. They are innervated by spinal nerves.
What do epaxial myoblasts form? What do thesemuscles do? What are they innervated by?
Epaxial myoblasts form the deep muscles of the back, they extend or straighten the trunk and are involved in lateral bending. These are innervated by the dorsal primary ramus of the spinal nerves.
erector spinae
muscles of the suboccipital triangle (connect the head and neck)
Whad do hypaxial myoblasts form? What do these muscles do?
The muscles of the lateral and ventral body wall. These flex and rotate the vertebral column directly (extrinsic back muscles).
Describe how some somites at different levels form appropriate muscle groups.
Cervical - strap muscle of the neck - diaphram.
Thoracic - intercostal, obliques, tranverse abdominis, rectus abdominis (flex the ventral body wall)
Abdominal/sacral - psoas, quadratus lumorum (flex the pelvic girdle)
Describe limb buds. How are they formed? When do they form? What are they made out of?
The limb buds are comprised of mesenchyme overlain by ectodermal tissue. The limb buds are formed when somitic mesoderm induces the proliferation of the overlying lateral plate mesoderm. Limb buds first appear in the fourth week of gestation as small elevations on the ventro lateral body wall.
How do limb muscles arise? Describe how they get to the limb bud and become myotubules.
The limb muscles arise from the lateral portions of limb level somites from precursors in the hypaxial region of the myotomes. These migrate into the limb bud where they differentiate into myoblasts and proliferate and fuse into myotubules.
As myoblasts migrate wthey condense into dorsal and ventral limb bud masses. What do each of these masses form and what are they innervated by?
The dorsal limb bud mass forms all the limb extensor muscles, the upper limb suppinators, and the lower limb abductors. These are all inervated by the dorsal branch of the ventral rami.
The ventral limb bud mass forms all limb flexors, the upper limb pronators and the lower limb adductors. These are all innervated by the ventral branch of the ventral rami.
Which of these is specified while cells are still in the somites, muscle specification or muscle fiber type (red vs white)?
Muscle fiber type (red/white).
From which somites do arm muscles form from?
Arm muscles form from myoblasts that migrate from C5-T1 somites.
From which somites do leg muscles form from?
Leg muscles form from myoblast that migrate from the L2-S3 somites.
Describe the formation of the pharyngeal arches.
During the 4th and 5th weeks neuroectodermal cells migrate to the rostrum of the embryo and develop a series of 4 arch-like protruberances on the lateral surface of the head.
What are cranial muscles of the head derived from? Why do they form this way?
Cranial muscles of the head are derived from unsegmented rostral somitomeres. This occurs because the cranial paraxial mesoderm, cranial to the first somite, forms a continuous strip with no overt segmentation.
Describe the formation of the cranial/paraxial mesoderm.
Cells originate adjacent to the neural tube and migrate to their destinations and aggregate into pre-muscle masses. Connective tissue elements lay down tracks over which the myoblasts migrate into the arches. Once in the arch they are innervated by cranial nerves which guide them to their destination. Each arch has its own cranial nerve with both afferent and efferent branches.
What is the cranial nerve for the 1st pharyngeal arch and what muscles does it serve?
Cranial Nerve V, the muscles of mastication
What is the cranial nerve for the 2nd pharyngeal arch and what muscles does it serve?
Cranial Nerve VII, Muscles of facial expression
What is the cranial nerve for the 3rd pharyngeal arch and what muscles does it serve?
Cranial nerve IX, stylopharyngeus muscle
What is the cranial nerve for the 4th pharyngeal arch and what muscles does it serve?
Cranial nerve X-XI, laryngeal musculature, and the pharyngeal plexus.
Describe how skeletal muscle fibers form from individual myoblasts. When is the fiber type sestablished?
Fusion of monnucleated myoblasts form multinucleated myotubules.
Expression of contractile proteins and arrangement of sarcomeres. (during this phase the functional fiber type is established)
How does connective tissue sheath play a role in myotubule formation?
The connective tissue sheath plays an instructitve role in the expression of proteins appropriate for different fiber types.
What are the two distinct classes of progenitor cells that muscle cells divide into during differentiation?
Primary muscle progenitor cells = "founder cells"
Secondary muscle progenitor cells = "Fusion - competent cells"
How do fusion-competent and founder cells interact?
Fusion competant cells aggregate around the region of founder cells and fuse with them to form myotubules.
What events does fusion involve?
Cell-cell recognition and attraction
Polarization (alignment)
Membrane fusion
What mediates groth and repair of skeletal muscle fibers in postnatal life?
Satellite cells or resident populations of mononuclear myogenicprecursors.
Where are satellite cells located?
Between the sarcolemma and basal lamina of muscle fibers and they divide at a slow rate to sustain self renewal and growth of muscle tissue.
What happens to satellite cells upon muscle cell injury?
Thye undergo proliferation under the influence of pax7. A small subset redeploy under the basal lamina for possible future use. Proliferative satellite progeny exit the cell cycle and begin differntiation uder the influence of MyoD and My5.
Frow what type of mesoderm are smooth muscle fibers primarily derived from?
Splanchnic mesoderm.
For each of these describe from what layer/mesoderm type they arise from.
Blood vessels
Intrinsic eye muscles
Mammary and sweat glands
GI - splanchnic mesoderm surrounding the primordial gut
Blood vessels - somatic mesoderm
Intrinsic eye muscles - ectoderm
Mammary and sweat glands - myoepithelial cells derived from somatic ectoderm
From what mesoderm does cardiac muscle develop from?
Cardiac muscle develops from splanchnic mesoderm surrounding the developing heart tube.
Do heart cells undergo fusion?
No, they are maintained as individual cells but do form intercalated disks and gap juctions.

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